This invention relates to level control and, more particularly, to a method and apparatus for maintaining a predetermined upper level in a fluidized particle bed.
It is desirable to deposit pyrolytic carbon coatings on certain objects. For example, uranium particles can be coated with a pyrolytic carbon which, in part, forms a pressure-retentive shell allowing the coated particles to be fabricated into fuel rods for use in nuclear reactors. Another important use for such coatings is for heart valve and other biomedical components because a pyrolytic carbon coating does not react with blood.
Pyrolytic carbon is usually deposited on an object by thermally decomposing gaseous hydrocarbons or other carbonaceous substances in vaporous form in the presence of the object. When pyrolytic carbon is deposited in a fluidized bed apparatus, one of the variables upon which the structure of the pyrolytic carbon will be dependent is the amount of available deposition surface area relative to the volume of the furnace enclosure wherein the deposition is occurring. Pyrolytic carbon which has a microstructure that has smaller growth features will be deposited when the relative amount of deposition surface area is fairly high. Thus, when relatively large objects; for example, objects having at least one dimension equal to 5 mm. or more, are being coated, an ancillary bed of small particles (usually of a size measured in microns) is included within the furnace enclosure together with the larger objects. This arrangement provides sufficient available total surface area to assure that pyrolytic carbon having the desired crystalline form will be deposited. In addition, the random motion of large objects in fluidized beds provides for a relatively uniform deposition of carbon on all surfaces.
However, whenever such submillimeter particles are being coated in a fluidized bed, the total surface area of the particles begins to increase significantly as the diameters of the pyrolytic carbon-coated particles grow. This change in the available deposition surface area in the fluidized bed will result in a change in the physical characteristics of the pyrolytic carbon being deposited if the other coating variables are held constant, e.g., coating temperature, gas flow rate and gas composition; and moreover, when the bed reaches some maximum size, it will collapse and thus limit the thickness of the carbon coating that can be deposited on levitated substrates under constant input conditions. Changes in the physical characteristics of the carbon deposited may be undesirable for any of a number of reasons.
It has been found that pyrolytic carbon having good structural strength and uniform physical properties can be deposited as relatively thick coatings upon relatively large objects in the accompaniment of particles if the available fluidized bed surface area is maintained relatively constant by withdrawing particles which have become enlarged in size as a result of coating and feeding smaller size particles into the deposition enclosure. Commonly assigned U.S. Pat. No. 3,977,896, the teachings of which are hereby incorporated by reference, is directed to this type of process for depositing pyrolytic carbon coatings. In that patent the flow of gaseous atmosphere is introduced beneath and generally centrally of the particle bed. Seed particles having relatively greater densities than that of the coating are introduced to the bed causing the coated particles to levitate where they can be removed through a withdrawal tube, the open end of which is positioned near the top of the bed. The rate at which the particles are removed is controlled by regulating the rate of flow of an inert gas up the tube. The seed particle input is at a constant rate, and the output is measured so that by varying the purge gas flow rate to regulate the output, a substantially constant bed total surface is achieved.
While such a coating process works well, the need for measuring the output and varying the purge gas flow rate in response thereto introduces certain complexities which it is desirable to avoid. It has been found that in many coating applications proper coating can be achieved by maintaining the bed at a predetermined level. The fluidized bed coating process requires an operating temperature of between 1200.degree. and 2000.degree. C. Prior art sensors for detecting bed level to control the rate of addition or removal are inoperable or unreliable under these fluidized bed operating conditions.